In general, the pressure-sensitive adhesive (hereinafter referred to as PSA) tapes or sheets are of the structure as illustrated in FIG. 1 or 2. More specifically, FIG. 1 is a schematically sectional view of a PSA tape or a PSA sheet free from any release liner, and FIG. 2 is a schematically sectional view of a PSA sheet including a release liner. In the drawings, reference numeral I is a PSA tape or sheet supporting substrate, II a release liner supporting substrate, III a PSA sheet surface member, IV a PSA layer, V a release layer, and VI is a release liner.
With the aforesaid PSA tapes or sheets, in most cases, silicone is generally used for the release layer thereof. However, the use of silicone offers the following problems:
(1) When the PSA mass used in the PSA tape or sheet has a relatively low adhesion (e.g., in the order of no more than 150 g/cm), the adhesive tape or sheet separates spontaneously from the release layer prior to the use thereof. This is due to excessive release properties of silicone. In consequence, the release layer loses its own function that it should protect the PSH layer may be contaminated.
(2) Difficulty is involved in marking the back surface of the PSA tape or the release (silicone)-treated surface of the PSA sheet free from any release liner with oil ink, water ink, ball-pointed pens, pencils, etc. due to their water- and oil-repellency.
It is noted that the release agents include, in addition to silicone, shellac, long-chain alkyl compounds, alkyd resin, etc. However, even when these agents are used, there is left much to be desired in respect of ink receptivity, although that receptivity is slightly improved, compared with the use of silicone. It is also difficult to obtain good releasability.
As a result of studies made to solve the aforesaid problems, it has been found that they are solved by a combination of modified starch and/or starch, which are used for the release layer in place of silicone, with PSA mass containing a minute and spherical elastomeric polymer, for which we have already filed a patent application (see a co-pending application Ser. No. 697,915, filed Feb. 4, 1985.